Yes my friends, we need SMELL-O-Vision in Stereo for this one!
I am much better being there to use my nose and to see where I might get the smells from, but I have an idea.
A Long time ago, My dad bought the master house of the property we were paying rent on. We moved into THAT house and rented the one we were renting. That rent paid for both houses and the acre of property! WOW! I say that, and wonder why it never sunk into my head to do that until the last 10 years! I was a very young boy!
Anyway, the house was nice, and had large rooms with heat vents everywhere!
We had that Smell in the main part of the house. Mom was a clean freak! Ned paint all over, and she made the drapes, when that used to be a thing! Inside was new-like and very clean. Still, something dead lingered for the longest time. It was a good thing the house was not insulated so well!
That next summer my dad replaced the old Octopus furnace in the basement with a brand new GAS furnace! He took out the many feet of duct pipe from the basement, and replaced all that too!
That winter we still had that dead animal smell.
Not sure how he came to check the heating ducts, but he managed to check one set of heating ducts that didn't flow so well. I guess you know where the story goes from here!
A large opossum managed to get into our ductwork, and died between the walls that were used as a plenum for the heating system!
Sorry for the boring story, but I never just give an opinion on how to solve something, without some background. Lots of armchair-quarterbacks these days, and I am not associated with heating/cooling businesses.
My suggestion, with the request that you let me know if I did help, is to have your HVAC ductwork system cleaned. Not just pay 200 bucks to have them change a filter! The professionals usually snake every vent down with a camera, a brush, and vacuum, to show you before and after too! And yes, if they offer a deodorize treatment of the sort, I would opt for it. It cant hurt!
Dead animal smells in the attic can come into the house through the unsealed vents too.
Dead animal smells WILL go away. Sure, YOU will know they are there, but nature has a way of cleaning stuff up.
Good luck,